Frequency modulation transmitter



Feb. 5, 1946. H F. MAYER 2,394,393

FREQUENCY MODULATION TRANSMITTER Filed May 26, 1943 5 FREQUENCY l6MULTOISLIER AMPLIFIER POWER QR AMPLIFIER BOTH FREQUENCY 8 MULEIRPLIER HAMPLIFIER 9 7 0R -3= 25; BOTH CRYSTAL 28 Z6, Z5 OSCILLATOR Inventor:Harry F. Mayer,

His Attorney.

Patented Feb. 5, 1946 FREQUENCY MODULATION TRANSMITTER Harry F. Mayer,Schenectady, N. Y., assignor to General Electric Company, a corporationof New York Application May 26, 1943, Serial No. 488,506

3 Claims.

The present invention relates to a freq ency modulation transmitter andis particularly concerned with a system for stabilizing or preventingdrift of the mean frequency of the transmitter.

In prior systems it has been customary to apply both signal andfrequency control voltages to a reactance tube in the resonant circuitof the transmitter oscillator with the result that the frequency controlvoltage prevents operation of the reactance tube at the optimum bias forlow distortion.

In the present transmitter this disadvantage is overcome by applying thefrequency control voltage to the reactance tube of a separate oscillatorand mixing the output of this oscillator with a frequency modulatedoscillator to produce a heterodyned output which is fed to thetransmitting antenna.

An object of my invention is to provide an improved arrangement forstabilizing the mean frequency of a frequency modulation transmitter.

The novel features which I believe to be char-.

acteristic of my invention are set forth with particularity in theappended claims. My invention itself, however, both as to itsorganization and method of operation, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription taken in connection with the accompanying rawing inwhichwFig. 1 is a diagram of a frequency modulation transmitterembodying my invention; and Fig. 2 is a diagram of a modification forsubstitution in the Fig. 1 transmitter.

Referring to Figfl of the drawing there is shown a modulator oscillatorI having a resonant circuit 2 in parallel with a reactance tube 3 havinga control grid 4 inductively coupled to the resonant circuit so that thevoltage applied to the control grid is in quadrature with the voltageapplied to the anode 5. The tube 3 draws a reactive current in an amountvarying with the bias applied to the control grid from an audio input 6and thereby causes modulation of the output frequency of the oscillatorin accordance with the audio signal. Because the output frequency of themodulator oscillator I is not controlled for frequency drift, no trimmercondenser is necessary in the resonant circuit 2 which permits themaximum L/C ratio for this circuit and the tube 3 can be adjusted foroperation at the bias for minimum distortion. The output of theoscillator accordingly has the maximum linear frequency modulation.

Associated with the modulated oscillator I is a similar oscillator Ihaving a resonant circuit 8 in parallel with a reactance tube 9 having acontrol grid I0 inductively coupled to the resonant circuit 8 to supplya voltage in quadrature with the voltage applied to the anode II. Thetube 9 draws a reactive current varying in magnitude with a frequencycontrol voltage applied through a conductor I2 to the grid III. Thefrequency of the oscillator varies in accordance with the frequencycontrol voltage. After amplification or frequency multiplication or bothin apparatus indicated at I3 and I4, the output of the oscillators I andI is fed to a mixer I5, the heterodyned output of which is fed through apower amplifier I6 to transmitting antenna I'I.

While the frequency radiated by the antenna is continually varying inaccordance with the the output of a crystal or other frequencystabilized oscillator II! to produce a beat frequency proportional tothe deviation of the mean frequency of the transmitter output from thefrequency of the crystal oscillator I9. Oscillations of this beatfrequency are amplified in an amplifler 20 and fed to a discriminator 2Iwhich produces a frequency control voltage proportional to the deviationor drift of the mean frequency of the transmitter output. The output ofthe amplifier 20, appearing in a resonant circuit comprising a condenser22 and an inductance 23, is coupled through a condenser 24 to themidpoint of an inductance 25 in parallel with a condenser 26. Thecondenser 24 and inductance 25, which comprise the discriminatorcircuit, are tuned to a frequency proportional to the desired meantransmitter frequency and are designed for maximum frequency stabilityand carefully temperature compensated. Because there is no mutualinductance between the inductances 23 and 25, the discriminator may belocated in any convenient position, for example in a temperaturecontrolled compartment. The quadrature voltage in the discriminatorcircuit, which in the 'conventional design is supplied by mutualinductance from the output of amplifier 20 is supplied by an unbalancingcondenser 21 connected between ground and the lower end of theinductance 25. The current flowing through the condenser 21 induces avoltage in the inductance 25 which is degrees out of phase with thevolt- 1 2 tage applied to the inductance when the frej quency of thevoltage is exactly equal to the resonant frequency of the discriminatorcircuit and which has a greater or less phase angle for frequenciesabove and below the resonant frequency.

The vector sums of the voltages in the inductf ance 25 above and belowthe midpoint are equal at the frequency for which the disciminatorcircuit is tuned and are unequal at frequencies above and below thatfrequency. These voltages are applied to diodes 28 and 29 connected inopposition across a condenser 30. The difference voltage appearingacross the condenser, which varies in magnitude and sign with thedeviation or drift of the mean transmitted frequency is fed through Ifless accuracy is required in the frequency stabilization, the crystaloscillator I9 andmi'xer I8 may be omitted and the transmitted frequencymay be fed to the discriminator through an amplifier 3| (Fig. 2)substituted in Fig. 1 in place I of the converter I8.

The automatic frequency control circuit per se utilizing thediscriminator apparatus 2| is described and claimed in a divisionalapplication, j Serial No. 597,039, filed June 1, 1945, and asj signed'tothe same assignee as plication.

the instant ap- While I have shown particular embodiments of myinvention, it will be understood that many 1 modifications may be madewithout departing s from the spirit thereof, and I contemplate by theappended claims to cover any such modifications I as fall within thetrue spirit and scope of my in- 1 vention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A frequency modulation transmitter, comprising two oscillators, meansfor heterodyning cordance with the signal, a discriminator responsive todeviation of the mean frequency, of said carrier from a desired value,and a reactance tube responsive to the discriminator for varying thefrequency of said other oscillator in the direction to maintain aconstant mean carrier frequency.

2. A frequency modulation transmitter, com-. prising a frequencymodulated oscillator, another oscillator having a reactance tube forvarying the frequency, means for heterodyning the output of saidoscillators'to produce a carrier, and a discriminator responsive to thedeviation of the mean frequency of the carrier from apredetermined valuefor producing a control voltage for controlling said reactance tube soas to maintain aconstant average carrier frequency.

3. A frequency modulation transmitter, comprising a source of highfrequency waves subject to slow variations in mean frequency, means forfrequency modulating said waves in accordance with a signal, anothersource of high frequency waves, means for heterodyning the outputs ofsaid sources to produce a carrier, and means responsive to slowvariations of the mean frequency Y of the carrier from a predeterminedvalue for changing the frequency of said other source to reduce saiddeviations. A

HARRY F. MAYER.

